The present invention relates to so-called "static trip" industrial circuit breakers, and particularly to a circuit breaker static trip unit equipped to provide ground fault protection as well as overcurrent protection for a distribution circuit.
To sense a ground fault condition, one approach is to compare the total current flowing in the distribution circuit from the source to the load with the total current returning from the load to the source. A ground fault exists if an imbalance or inequality in these currents is found, since some of the current leaving the source is then flowing through the ground fault and returning to the source via an extraneous ground circuit path and the ground return conductor connecting the source to ground. One way of detecting this current imbalance is to link the distribution circuit conductors with a differential current transformer core and look for a current induced in a secondary winding thereof by residual flux flowing in the core. An alternative approach to detecting this current imbalance is to connect the secondary windings of individual phase current transformers and a neutral current transformer, if a neutral conductor is present, into a summing circuit. If the vectorial summation of the secondary currents is zero, no ground fault conditions exist. However, if the vectorial summation of these secondary currents is not zero, then a ground fault condition is indicated. The differential secondary current is then sensed to develop a ground fault signal for processing pursuant to initiating a ground fault trip function.
As an alternative approach to ground fault sensing, rather than looking for an imbalance in the currents flowing to and from the circuit loads in order to indirectly monitor the matnitude of any ground fault current, the magnitude of the ground fault current may be sensed directly by coupling a current transformer with the source ground return conductor. The ground fault signal appearing in the secondary of this current transformer is then processed pursuant to initiating a ground fault trip function.
It is accordingly an object of the present invention to provide an improved circuit breaker static trip unit having both overcurrent and ground fault tripping capabilities.
Another object is to provide an improved ground fault trip mode network for incorporation in an overcurrent static trip unit utilized in industrial circuit breakers.
An additional object of the present invention is to provide a ground fault trip mode network utilizing a miniature current transformer to sense ground fault occasioned differential current flowing in the input section of an overcurrent static trip unit.
Yet another object is to provide a ground fault trip mode network of the above character which is capable of providing time-current response characteristics conducive to improved coordination with the response characteristics of circuit protective devices upstream and downstream thereof.
A further object is to provide a ground fault trip mode network of the above character, which is equipped to accommodate zone selective interlocking with upstream and downstream circuit protective devices.
A still further object is to provide a ground fault trip mode network of the above character which is capable of stand-alone application in ground fault protective devices and systems independently of an overcurrent protective device.
An additional object of the present invention is to provide a ground fault trip mode network of the above character which is efficient in construction, reliable in operation, and versatile in application.
Other objects of the invention will in part be obvious and in part appear hereinafter.